In the heat processing of materials, a work product is usually conveyed through an elongated furnace which can be divided into respective zones, each of which may have its own temperature and gaseous environment. The work product is often conveyed through the furnace by means of a woven metal belt which is moved along the furnace hearth. At elevated temperatures, generally above 1,110.degree. C. such as required for the firing of ferrites and titanites and for the sintering of powder metals and nuclear fuel elements, such metal conveyor belts cannot be employed since the metal will soften or melt at these elevated temperatures. In addition, even at temperatures at which the metal retains its structural integrity, such belts are often a source of particle contamination.
In order to accomplish heat processing at temperatures at which metal conveyor belts are unusable, furnaces have utilized push-rod systems in which a row of product carriers are conveyed through a furnace by means of a reciprocating push rod located at the entrance end of the furnace. The product carriers in such pusher systems must be sufficiently strong to transmit the compressional force applied by the push rod, resulting in a thermally massive carrier sensitive to thermal shock, and of sufficient mass to require a considerable heating and cooling period. Additionally, in such push-rod systems, the product carriers are usually in sliding contact with the muffle hearth, giving rise to abrasion which can cause contamination and consequent degradation of the furnace environment.
To overcome the limitations of push-rod conveyor systems, furnaces have been developed which include a furnace hearth movable with cyclic vertical and horizontal motion to transport product carriers supported thereon in step-wise fashion through the furnace. The movable hearth is usually supported for vertical and horizontal motion by respective pluralities of hydraulic cylinders disposed below the furnace chamber to provide lifting and lowering motion and at each end of the furnace for providing lateral movement of the hearth. These driving elements must be operated in a timed sequential manner to provide intended conveyor motion, and in practice, sequential operation of the plurality of driving elements is difficult to maintain. As a result, precise and consistent transport of a work product through a furnace is not easily achieved and the accuracy of the thermal processing can consequently be impaired. A synchronous operation of the plurality of driving elements can also cause bending moments to be applied to the furnace hearth, which can result in damage to the hearth. In addition, the driving elements below the furnace can be damaged by the high furnace temperature and are not easily accessible for maintenance.